The geological condition occurs when underground nuclear blasts cause the surrounding rock to become weak and permeable.

The 3 September test caused the USGS to register a tremor of magnitude 6.3, with a smaller, second earthquake being measured at the site, about eight minutes later.

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Video:Trump: US ready for N Korea 'military option'

The USGS said the second quake was indicative of a collapse and it recorded a further seismic event of 3.1 magnitude on the site on 23 September.

The 38 North website, which reports on issues involving North Korea, said the effects of the latest blast could potentially extend as far as 1.4km from the detonation point.

But it said they probably won't stop Mount Mantap being used for further tests.

Other specialists, however, have said the effects on the mountain mean the Punggye-ri site may not be used for much longer to test nuclear weapons.

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Video:The frontline against North Korea

Meanwhile, Seoul and Beijing have agreed to move beyond a year-long stand-off over the deployment of the US anti-missile THAAD system in South Korea.

The dispute has been devastating South Korean businesses that rely on Chinese consumers and the announcement comes just days before Donald Trump begins a trip to Asia, where the North Korean nuclear crisis will take centre stage.

In Japan, where tensions have been raised as a result of the North Korean nuclear and missile tests, Prime Minister Shinzo Abe has met NATO Secretary-General Jens Stoltenberg.

They discussed cooperation in addressing security challenges, with Mr Stoltenberg telling North Korea afterwards it "must abandon its nuclear ballistic missile programmes and implement a complete denuclearisation of the Korean Peninsula".